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IMPACT OF CHAR AND ASHFI N SON PORO US CERAMIC FILTER LIFEM. A. Alvin

Westinghouse Electric CorporationScience Technology Center

Pittsburgh, PA 15235Keywords Char, ash, filtration.ABSTRACTAlthough frequently inert, char and ash fines can potentially have a deleterious impact on the life.of porous ceramic filters that are currently being utilized in advanced coal-fired applications. Thispaper reviews several of the char and ash related issues that have been encountered in variousWestinghouse Advanced Particulate Filtration systems which limited filter life.INTRODUCTIONDuring the past 10-15 years, Westinghouse has been involved in the development of AdvancedParticulate Filtration (APF) systems for removal of char and ash fines that are released during coaland biomass gasification or combustion, Initially individual porous ceramic filter elements wereinstalled in slipstreams at the Kellogg Rust-Westinghouse (KRW) fluid-bed gasification testfacility in Madison, Pennsylvania, and at the New York University combustion test facility inWestbury, New York. Recently these efforts have been expanded to the us of multiple clusterarrays containing 384 or 748 fdter elements within a common pressure vessel at the AmericanElectric Power (AEP) pressurized fluidized-bedcombustion (PFBC) demonstration plant inBrilliant, Ohio, and at the Sierra Pacific integrated gasification and combined cycle (IGCC) testfacility in Reno, Nevada.Numerous phase changes and generally a loss of material strength were encountered during use ofthe monolithic first generation filters under PFBC conditions at AEP, as well as during operationin AhlstromlFoster Wheeler's pressurized circulating fluidized-bed combustion (PCFBC) testfacility in Karhula, When catastrophic failure of the first generation monolithic filterelements occurred, it generally resulted from thermal fatigue or shock of the oxide-basedalumindmullite matrices, or from creep crack growth of the nonoxide-based clay bonded siliconcarbide filter materials. Advanced second generation, fracture toughened, fiber reinforced filterelements are currently being developed and insN,ed>n W-APF systems in an attempt to mitigatecatastrophic failure of the porous ceramic matrixauring process operation. The stability and/orchanges within the second generation filters materials are being evaluated in on-going testprograms at the Westinghouse Science and Technology Center in Pittsburgh, Pennsylvania. 'Although the composition and morphology of the first and second generation filter materialschange during exposure to advanced coal-fired and biomass process environments, the porousceramic filter elements generally remain intact, surviving exposure to the process temperature andgas chemistry. Unfortunately failure and performance limitations (Le., reduced gas flowpermeability; decreased particle removal efficiency; etc. have been encountered due to ash relatedissues as 1) bridging; 2) filter bowing; 3) inner wall blinding; 4) inner bore plugging; 5 )wedging of fmes between the ceramic filter flange and metal holder mount; 6) auto-ignition; and7)membrane debonding in alkali-laden systems. The following section provides an overview ofWestinghouse's field experience where filter life and performance have been l i mte by thedeposition and adherence of char and ash fines.FIELD EXPERIENCEDuring the early test campaigns at AEP, the Westinghouse APF systemwas challenged with inletdust loadings of 600 ppmw. consisting primarily of h e sh particles with a mass mean particles i z e of 1-3 pm s a result of the sm ll particle size and the tendency for the particles to sinter.dust built-up between filter elements, and along the dust sheds and plenum support pipes, formingcompact bridges. Ash bridging lead to bowing and distortion of the clay bonded silicon carbidecandles at process operating temperatures of 620-790C. as well as random catastrophic failureoffilters during plant startup and shutdown. Post-test inspection of the filter arrays indicated theclose proximity and/or contact of the bottom closed end caps of adjacent bowed candles, and thefresh fractured surfaces at the base of the filter flanges.

American Electric Power- ressurizedFluidized-BedCombustion

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catastrophic failure. s a result, efforts fostered the development of advanced high temperature,creep resistance binder-containing. clay bonded silicon carbide candle filters.

Foster Wheeler- arbonizer and Combustor est FacilitiesIn 1992, Westinghouse installed and operated an APF system at the Foster Wheeler carbonizertest facility in Livingston, New Jersey. Twenty-two alumindmullite candles formed the filterarray. Initially one element failed, and char fines were. back pulsed into the inner bore of theremaining filter elements. During startup in an oxidizing environment, combustion of the f i nesresulted, and catastrophic failure of several of the alumindmullite filter elements occurred.Aiii luugh aiiemare Frsr or second generation riiter elements had not been subjected to similaroperating conditions, limited survival of these materials to withstand auto-ignition is expected.In 1993 an alternate array of candles was installed in the W-APF system and operated undersecond generation PFBC conditions at the Foster Wheeler. During a -210 hour test segment, thecyclone plugged and the filter vessel was fded with ash fines, causing failure of several of thefilter elements. Post-test inspection of the array indicated that a tenaciously bonded sodium-potassium sulfate eutectic melt formed along the outer surface of the filter elements, which thenserved as the collection site for the adherence of CaSO,, CaO and CaC03 fmes. ' The. formationof the eutectic melt readily caused debonding of the membrane along the outer surface of the claybonded silicon carbide filters. The alumindmullite candles were virtually unaffected. Additionalreactions of the binder phase in the clay bonded silicon carbide filter elements with gas phasealkali released in the Foster Wheeler second generation PFBC effluent stream were also observed.The deleterious impact of auto-ignition of char fines, as well as the formation of the mixed alkalieutectic phase and bed carry-over require. more stringent process control in order to achievesuccessful long-term operation of the ceramic filter elements in the advanced particulate removalsystems.

Texaco- ntrained-Bed G asificationWestinghouse utilized cross flow filters during the early test campaigns at the Texaco entrained-bed gasification test facility in Montekllo, California. Failure was encountered when the stackedplate alurnindrnullite cross flow filters delaminated, and when char finesbecame wedged betweenthe cross flow flange and metal filter holder mounts. Wedging and failure along the flange hadalso been observed during long-term durability testing of the cross flow filters under simulatedhigh temperature, high pressure, PFBC conditions at the Westinghouse test facility in Pittsburgh,PennsylvaniaDuring the later test campaigns, clay bonded silicon carbide and alumindmullite candle filterswere retrofitted into the Westinghouse filter vessel at Texaco. After -400 hours of operation, thecandles were removed, and were observed to be. coated with a tenaciously bonded dust cakelayer. Removal of the FeO OH), FeA1204eMched cake from the outer surface of the filterelements was extremely difficult, which ultimately lead to the removal of the membrane of the claybonded silicon carbide candle filters. '

Biomass G asificationIn 1994 and 1995 Westinghouse operated a twelve filter element array at the IGT biomassfluidized-bed gasification test facility in Chicago, Illinois. Post-test inspection of the clay bondedsilicon carbide filter elements after -21 and -30 hours indicated that either an -5.3 mm brown or-1-2 mm black dust cake layer remained along the outer surface of the filter elements. Theh 3 0 4 Fez03 C, K, nd SiOzenriched dust cake layer which was easily removed from the filtersgenerally retained the contour of the candle body. ' Debonding andor removal of the outermembrane of the clay bonded silicon carbide candles was not observed after short-term operationin the 675-916C biomass gasification environment.FILTER ELEMEN T LIFEAND REGENERABILITYPorous ceramic filter elements can be utilized for extended service operation as demonstrated byWestinghouse during conduct of the five test campaigns at AEP where several surveillance claybonded silicon carbide candles successfully survived 5855 hours of operation. At AEPWestinghousealso demonstrated the regenerability of the clay bonded silicon carbide candles byinitially brushing the ash cake layer from the outer surface of the filter elements, followed bywater washing and drying. Nearly complete gas flow permeability was recovered for the filter

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elements in t h i s manner. If ash blinding occurred along the innerwall or fines formed a compactinnerbore plug, complete regenerability ofthe elements was not possible.FILTER ELEMENT CONSTRUCTIONDuring field operation, the texture or roughness of the outer surface as well as changes in thewall thickness of the filter elements dramatically impact the adherence andor removal of the dustcake layer. As the texture or roughness of the surface increases. residual ash cake thickness tendsto increase, leading to a reduced gas permeability through the filter wall. An optimal constructionof the filter element is a uniform wall thickness along the length of the body and closed end capareas. In this manner uniform removal of the dust cake layer can be accomplished during pulsecleaning.FUTUREEFFORTSWestinghouse will continue to explore the impact of char and ash fines on the stability of theporous ceramic filters during future testing at the Sierra Pacitic Pinon Pine gasification test facilityin Reno, Nevada, and at the SouthernCompany Services integrated gasification and combinedcycle (ICCC) test facility in Wilsonville, Alabama, and at the Pacific International Center for HighTemperature Research (PICHTR) biomass gasification demonstration plant in Hawaii. Testing iscurrently on-going at the Foster Wheeler/Ahlstrom test facility in Karhula, Finland, whereadvanced first and second generation monolithic and fi er reinforced filter elements are beingsubjected to 900CPCFBC test conditions. As a result of these efforts, the ultimate viabiity,performance and robustness of the fmt and second generation porous ceramic filter elementswillbe challenged under a relatively wide gamut of process operating conditions ll of which arerequired to define successful long-term operation of the advanced high temperature particulatefiltration systems.REFERENCES1 M. A. Alvin, Advanced Ceramic Materials for Use in High Temperature Particulate Removal

Systems, 1995AIChE Annual Meeting, Miami, FL, November 17,1995.. 2. M. A. Alvin, R. E. Tressler, T. E. Lippert, E. S . Dim, and E. E. Smeltzer, Durability ofCeramic Filters, Coal-Fired Power Systems 94- dvances in IGCC and PFBC Review

Meeting, Morgantown,W V June 21-23, 994.3. M . Alvin, T. E. Lippert, E. S . Dim, and E. E. Smeltzer, Filter Component Assessment,Advanced Coal-Fired Power Systems 95Review Meeting, Morgantown, WV. June 21-29,1995.4. M A. Alvin, Advanced Coal-Fired and Biomass Ash and Char Formations, 12thInternational Pittsburgh Coal Conference, Pittsburgh, PA, September 11-15.1995,5. M. A. Alvin, CharacterizationofAsh and Char Formations in Advanced High Temperature

Particulate Filtration Systems, Fuel Processing Technology, Vol. 44,Nos. 1-3,September1995, p. 237-283.

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